1. Field of the Invention
The invention relates to the field of photoacoustic spectroscopy for the detection of impurities in liquid media. It proceeds from a photoacoustic flow measuring cell according to the preamble of claim 1.
2. Discussion of Background
In the offshore conveyance of crude oil, so-called separating tanks are used, in which the various phases (sand, water, oil and gas) occurring during drilling or conveyance are separated by virtue of their density differences and are discharged in separate line systems. In this case, even slight oil impurities in the waste water may lead to unacceptable environmental pollution, with the corresponding consequences as regards cost. Typical concentration limit values for oil in water are 40 ppm in the case of direct disposal into the ocean and 100 ppm-5000 ppm in the case of reuse as process water, for example when pumped back into the oil source. In order to monitor the limit values, therefore, it is necessary to have detection-sensitive and reliable oil residue detectors.
High pressure separating tanks have recently been developed, which are suitable for operating on the ocean floor a few hundred meters below the surface of the ocean. Conveyed and already separated oil can then be pumped to the surface of the ocean with a much lower expenditure of energy. Such separating tanks are exposed to very high pressures of 60-180 bar, specifically, from outside, to the water pressure at the bottom of the ocean and, from the inside, to the pressure of the conveyed crude oil, as well as to high temperatures of 50-120.degree. C. An oil residue detector must be operational for a period of years, and without requiring maintenance, under these difficult operating conditions, since operating failure and premature replacement would incur high costs.
Photoacoustic sensors for trace analysis in liquids and, in particular, of oil in water are known from the article by H. A. MacKenzie et al., "A laser photoacoustic sensor for analyte detection in aqueous systems", Sensors and Actuators B, 11, 213-220 (1993). The photoacoustic measuring method is based on converting optical energy into acoustic energy by means of foreign molecules in liquids. Intensive pulsed laser radiation is transmitted to a measuring cell, is absorbed by the molecules to be detected and is converted into thermal energy by relaxation without radiation. Local heating in the absorption region leads to thermal expansion and to the radiation of an acoustic pressure wave. The frequency of the acoustic wave is determined by the repetition rate or modulation frequency of the laser radiation. The excitation efficiency is proportional to the laser pulse energy absorbed, to the coefficient of thermal expansion and to the sound velocity and is inversely proportional to the heat capacity. Advantageously, the wavelength is selected in such a way that absorption is low in the medium and high in the foreign molecules. The degree of detection of impurities is a plurality of orders of magnitude better in the case of high laser pulse energies and in short measuring cells than in the case of direct infrared, transmission or reflectance spectroscopy.
In measuring cells for aggressive or coating-forming analytes, the contamination of sensor components, such as, for example, the optical apertures or else the acoustic detectors, presents a considerable problem. U.S. Pat. No. 5,125,749 discloses a photoacoustic flow measuring cell for liquids, in which a sensor head is equipped with a fiber optic light supply, an optical aperture, an acoustic pressure transducer and an electric line for signal transmission to evaluation electronics. The sensor head and, in particular, the optical aperture are exposed directly to the liquid stream. The sensor is therefore highly susceptible to deposits on the optical aperture. Specifically, for use in a separating tank, it must be expected, because of the extreme and variable temperature and pressure conditions, that oillike and waxlike layers will result in pronounced coating formation.
U.S. Pat. No. 5,339,674 discloses a photoacoustic flow measuring cell for chemically aggressive gases. The optical aperture and the microphone are separated from the photoacoustic interaction zone by an elongate pipe and are additionally protected by an inert protective gas against being contaminated by the measurement gas. In this case, the pipe through which the gas flows serves as an optical transmission stage and, at the same time, as an acoustic waveguide. However, this measuring arrangement is unsuitable for liquid measuring cells because of the acoustic impedance jump between the liquid and the protective gas.
The prior art also specifies optical methods for the contactless detection of ultrasonic waves at surfaces. The article by Jean-Pierre Monchalin, "Optical Detection of Ultrasound", IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control 33 (5), 485-499 (1986) describes optical sound detectors which are based on beam deflection or interferometric interference formation. Monolithically constructed interferometers suitable for everyday use, with integrated laser diode and photodiode, are nowadays obtainable commercially.